An electrophotographic printing machine forms images by the electrophotography as described below. First, a photosensitive layer formed on the surface of the photoreceptor is uniformly charged with a single polarity. Then, an electrostatic latent image is formed on the surface of the photoreceptor through the process of exposure. Further, fine colored particles (toner) charged by applying thereon a charge having a polarity opposite to the charge on the photosensitive layer are made to adhere the electrostatic latent image by electrostatic force, thereby forming images.
In the electrophotographic printing machine thus described, conventionally a corona discharger has been used as a charger for uniformly charging with a single polarity the photosensitive layer formed on the surface of the photoreceptor. However, in the case of adopting the corona discharger, an application of high voltage is required for a wire electrode when charging the photosensitive layer. This presents the problem since a power supply for applying voltage to the wire electrode becomes larger in size. Moreover, with the corona discharger, an erosion of the components of the machine and the deterioration of the surface of the photoreceptor are likely to occur due to ozone produced in the process of corona-discharging the photosensitive layer. This results in the problem by making unclear and fuzzy the images, or by adversely affecting the human body, etc.
In order to counteract the above-mentioned problems, the contact charger provided with an electrically conductive contacting component in contact with the photosensitive layer formed on the surface of the photoreceptor has been recently proposed. With this contact charger, the photosensitive layer formed on the surface of the photoreceptor is charged through the contacting component by applying DC voltage to the contacting component.
For the contacting component designed for the contact charger, for example, a charge roller 41 shown in FIG. 5 may be used. The charge roller 41 is arranged such that an electrically conductive elastic layer made of a flexible material such as a silicone rubber including a carbon is formed on a surface of an electrically conductive cylindrical roller base. The charge roller 41 is freely rotative about an axis parallel to a rotation axis of a photoreceptor 42. Further, the charge roller 41 is in contact with the surface of the photoreceptor 42 with a predetermined nip width. The charge roller 41 rotates in conjunction with the rotation of the photoreceptor 42.
A power supply (not shown) being connected to the roller base of the charge roller 41 applies DC voltage to the surface of the photoreceptor 42 through the charge roller 41.
Here, the photoreceptor 42 is designed so as to have the photosensitive layer on the surface thereof having a property as an insulator when it is not exposed, on the other hand, having an electric property which makes the exposed portion thereof electrically conductive when it is exposed. The base of photoreceptor 42 is made of an electrically conductive material and connected to ground so that a charge on the photosensitive layer is released to ground through the process of exposure.
However, the arrangement of the discussed contact charger results in the following problems when a defect exists such as a pin hole on the photosensitive layer formed on the surface of the photoreceptor 42 due to a deterioration thereof.
Namely, at the portion where the defect exists, there may have a continuity between the surface of the charge roller 41 and the base of the photoreceptor 42 as an electric conductor since the charge roller 41 and the photoreceptor 42 are in direct contact with one another. This means that excessive current flows between the charge roller 41 and the base of the photoreceptor 42, and thus voltage of a power supply suddenly drops. As a result, an improper charge occurs in an axis direction at the portion where the defect exists (drop in the charged potential) on the photosensitive layer formed on the surface of the photoreceptor 42, thereby presenting a problem of an irregular image.
In order to counteract the problem of an improper charge due to a leakage of current from the portion where the defect exists on the photosensitive layer, there is an idea of increasing a resistance of the contacting component. However, if the resistance of the contacting component increases, it is difficult to apply sufficient charges to the entire surface of the photoreceptor.